Integrated circuits that are fabricated using the same lithography masks can be uniquely identified by embedding a unique identifier in the chip, such as a serial number embedded in the chip by the manufacturer. Another example of generating a unique identifier is to incorporate an array of transistors in the chip, measure the threshold voltages of the transistors in the array, and output the measurements as the identifier. For a given number of chips made from the same lithography masks, if the number of transistors in the array is large enough, the identifiers generated from the array will be unique. Due to process variations in the fabrication of the chip, no two chips will have arrays of transistors whose threshold voltages are exactly the same.
A unique identifier that is secretly embedded in a chip can be used to authenticate the chip. Authentication means proving to a user that the chip is not a counterfeit, or proving that certain processing results are processed by the chip and not some other chip. An example of a unique identifier is a secret key embedded in a smartcard. A card reader reads the secret key from the smartcard and compares it with a secret key previously stored in a database. If there is a match, the smartcard is authenticated, and the card reader can proceed to transact with the smartcard. The secret key needs to remain secret so that an adversary cannot duplicate the key and falsify identity.
An adversary may probe the chip to attempt to find the secret key using invasive methods, e.g., removal of the package and layers of the integrated circuit, or non-invasive methods, e.g., differential power analysis that attempts to determine the key by stimulating the integrated circuit chip and observing the power and ground rails. To prevent physical invasion of the chip, sensing circuitry may be included in the packaging of the chip to detect intrusion and erase sensitive information upon detection of intrusion.